--- /dev/null
+#if 0
++----------------------------------------------------------------------+
+| Question: My filter design package gives me decimal fractional |
+| numbers as output for my FIR filter coefficients. |
+| How do I convert these decimal fractions into the |
+| DSP56200's format? |
++----------------------------------------------------------------------+
+
+ Answer:
+ It's fairly easy to convert decimal fractions into the
+ data format required by the DSP56200. The DSP56200 coeffi-
+ cients are represented as 24-bit signed, fractional, 2's
+ complement numbers as shown in Table 1.
+
+ TABLE 1 - Representations for FIR Filter Coefficients
+ -----------------------------------------------------
+
+ Decimal Hexadecimal
+ Fraction 24-Bit Binary Value Coefficient
+ -------- ------------------- ----------
+
+ 0.75 0.1100000 00000000 00000000 60 00 00
+ 0.5 0.1000000 00000000 00000000 40 00 00
+ 0.25 0.0100000 00000000 00000000 20 00 00
+ 0.125 0.0010000 00000000 00000000 10 00 00
+ -0.125 1.1110000 00000000 00000000 F0 00 00
+ -0.75 1.0100000 00000000 00000000 A0 00 00
+
+ Each 24-bit coefficient is separated into three bytes
+ corresponding to the high, middle, and low bytes. The high
+ byte is written to the DSP56200 at address 0A (hex), the
+ middle byte to address 0B (hex), and the low byte to address
+ 0C (hex).
+
+ Conversion Procedure:
+
+ Step 1: Multiply the decimal fraction by 8388608.0 (decimal).
+ Note that 8388608 = 2 raised to the 23rd power.
+ Step 2: Truncate or round the result into an integer.
+ Step 3: Logically AND this integer with 00FFFFFF (hex).
+ Step 4: Write the result to an output file as a hex integer.
+
+ It is easy to write a program to perform this conversion on
+ a set of coefficients. If done by computer program, make
+ sure that all integer variables are represented with at
+ least 24-bits. An example of a short "C" program is
+ included at the end of this answer.
+
+ Things to watch for:
+
+ (1) Avoid letting a coefficient value be exactly 800000
+ (-1.0 in a fractional system). If this coefficient
+ is multiplied by a data sample with value -1.0, the
+ result is -1.0 instead of +1.0 as expected. This is
+ because +1.0 cannot be represented in a signed, 2's
+ complement, fractional system.
+
+ (2) The filter coefficients must be carefully selected
+ to prevent overflow. If there is a possibility of
+ overflow with a set of filter coefficients, then all
+ coefficients must be scaled by a constant value. The
+ DSP56200's 24-bit coefficients allow plenty of room
+ for scaling. If 12-bit input data samples are used
+ in a system, the potential for overflow is greatly
+ reduced if the samples are right justified and sign-
+ extended four bits before they are sent to the
+ DSP56200.
+
+"C" Program:
+#endif
+ /****************************************\
+ ** DECIMAL FRACTION TO HEX CONVERSION **
+ \****************************************/
+
+/******************************************************************\
+* This program converts one decimal, fractional number to a hex *
+* number which can be written to the DSP56200's Coefficient RAM *
+* Access registers. It prompts the user for a decimal fraction *
+* and returns the corresponding hex number. *
+* *
+* Examples of Program Results (useful for checking programs) *
+* Inputs Outputs *
+* ------ ------- *
+* 0.750 => 600000 *
+* 0.500 => 400000 *
+* 0.250 => 200000 *
+* 0.125 => 100000 *
+* -0.125 => f00000 *
+* -0.750 => a00000 *
+* 0.00784313678741455 => 010101 *
+* -0.00784313678741455 => fefeff *
+* *
+* Note: The program assumes that the variable type "long" is an *
+* integer which is at least 24-bits wide. *
+* *
+* Also: The DSP56200 cannot use any coefficient value with a *
+* magnitude of 1.0 or larger. All coefficients must be *
+* signed, fractional quantities. *
+\******************************************************************/
+
+main()
+{
+ double fractnum; /* double precision floating pt */
+ long hexval; /* long integer */
+
+ for(;;) {
+ /* Read 1 Decimal Floating Point Number from the Keyboard */
+ printf("Enter the decimal fraction: ");
+ scanf("%lf", &fractnum);
+
+ /* Convert to a Hex Integer which can be used by the DSP56200 */
+ hexval = (long) (8388608.0 * fractnum);
+ hexval = 0x00ffffffL & hexval;
+
+ /* Write the Hex number out to the Terminal */
+ printf("DSP56200 Coefficient = %06lx\n", hexval);
+ }
+}
projects / a56.git / blobdiff